This invention relates to the art of measuring angular displacements relative to a reference line or plane. More particularly, the present invention relates to devices for measuring horizontal angular displacement, vertical angular displacement or both. The invention is particularly useful for indicating angular displacement relative to the polar axis line of an external magnetic field such as is involved in compass operations and also is especially useful for indicating vertical angles relative to the horizontal as is involved in inclinometers or the like.
Prior compass devices have been known for many years. Basically, the compass apparatus includes a magnet which is located in a horizontal plane with a low friction support such as a needle bearing so that the magnet can align itself with the magnetic field associated with the surface of the earth. Pointers and graduated dials are associated with the floating magnet so as to permit measurement of the angular displacement for a given sighting relating to magnetic orientation of the earth's field. For many purposes, visual reading of the compass dials has been sufficiently accurate and are still extensively employed using essentially the same balancing apparatus that has been known for many years.
Various efforts have been directed towards development of improvements in the accuracy of reading of such devices and expanding the applications thereof. For instance, bubble levels have been added to the compass frame to assist in orienting the device horizontally in a more accurate position. In addition, adaptation of compass type devices to vertical angular measurements have also been employed. Such devices are sometimes employed as transits and require some additional means for determining a horizontal reference such as through a supplementary bubble level so that the angle of vertical inclination for a given sighting can be determined visually through cross-hair sights or the like. In addition to the well known application as a navigational aid, these combined compass/transit devices have been particularly useful for rough and preliminary surveys by a wide variety of disciplines such as geologists, civil engineers, foresters, mining engineers and the military. For some such applications, the prior devices suffer from several disadvantages such as reliable accuracy of visual reading especially in an environment of poor lighting as is encountered in mines and at night.
There have been some efforts toward digitizing the readout of compass-type devices. For instance, U.S. Pat. No. 3,746,842 by Fowler shows a gimbal mounted magnetic compass assembly which has a digitally encoded card attached for movement with the magnets. The digital code on the card is read by an arrangement of a light source and a series of light sensitive detectors to control the loading of clock pulses into counter circuitry so that the instantaneous orientation of the magnets can be read by electronic circuitry. U.S. Pat. No. 3,746,842 further suggests including a specially configured strobing control track on the card. Similar coded disc position indicating devices are also shown in U.S. Pat. Nos. 3,772,503 by Fowler and 3,833,901 by Fowler. The use of direct digital displays of the detected digital position defining codes has been suggested in U.S. Pat. Nos. 3,825,823 by Rovner and 3,746,843 by Wesner.
Despite the improvements in the state of the compass/transit/inclinometer art, there has been a continuing need for a device which can be either hand held or used in conjunction with supporting apparatus such as tripods and the like which will provide a repeatably accurate digital indication of angles of displacement relative to an external magnetic field or a horizontal plane. Further, no prior art devices permit the use of a single pivotally mounted assembly and support which accommodates both a horizontal displacement measurement and a vertical displacement measurement. Still further there is need for such a device which will provide for a digital readout to directly illuminate a visually readable numeric display or provide a potential digital read-out for other apparatus or both.